This invention relates to a multiplexing device for sending multiplexed signals through a digital line, and to a high performance digitally multiplexed transmission system in a switching system.
FIG. 1 shows an example of a voice multiplexing device or a switching system, and FIG. 2 shows an example of a multi-media multiplexing device or a switching system. In FIG. 1, voice data from a phone terminal 1 are applied to a relay transmission network 3 through a voice multiplexing device/switching system 2. The voice data are transmitted to a receiving phone terminal 5 through a voice multiplexing device/switching system 4. The relay transmission network 3 comprises a digital relay line and a voice multiplexing device/switching system for a relay.
In FIG. 2, transmission information inputted to a multi-media multiplexing device/switching system 9 from the phone terminal 1 through PBX 6, from a data terminal through a packet switching system 7, and from a TV conference terminal through an image switching system 8 are applied to a relay transmission network 10. Data are outputted to a PBX 12, a packet switching system 13, or an image switching system 14 through a multi-media multiplexing device/switching system 11.
In such voice multiplexing devices, multi-media multiplexing devices, etc., a time-divisional multiplexing (TDM) method is usually adopted. However, another method such as a digital speech interpolation (DSI) multiplexing method or a digital data interpolation (DDI) multiplexing method may also be adopted as a statistical multiplexing method for efficient transmission. A DSI method detects a silent section in voice data and transmits only speech sections. A DDI method supplements information from queuing data communication media such as a packet switch system before performing a multiplexing operation.
FIGS. 3A and 3B show examples of band allocations. FIG. 3A is for a conventional TDM multiplexing method and FIG. 3B is for a statistical multiplexing method. In FIG. 3A, a band is allocated to each voice channel and the object is to get a higher performance voice coder with a view to improving sound quality within the allocated band. By contrast, in FIG. 3B, bands are flexibly utilized by a plurality of voice channels, thus extending the equivalent band per channel and improving transmission quality.
FIG. 4 shows a band allocation using a DSI method. In this method, a band is flexibly allocated depending on whether data comprise speech or silence by detecting silent sections. Where there are a greater number of voice calls more bands are allocated and sound quality is improved.
As described above, in a conventional TDM multiplexing method, for example, the same number of bonds allocated to the maximum number of calls are also allocated to a small number of calls. The problem with this method is that sound quality is kept constant as for the maximum number of calls, and the average band cannot be extended by the flexible allocation of bands according to the voice level.
In the DSI method, silent sections are detected and no information of those sections is transmitted. That is, the band for those sections is considered to be zero, thus improving the transmission efficiency. However, when a line is congested, the whole coded information for one voice channel is either transmitted "as is"0 or discarded completely. This causes the problem that sound quality greatly deteriorates for a channel where the whole coded information is discarded completely.